Unit 2 Day 10 Today’s Topics: Ch 7 Muscular System Muscle Structure Muscle Physiology

Muscular System Muscles utilize chemical energy from the breakdown and metabolism of food to perform useful work. There are three kinds of muscle cells: skeletal, smooth, and cardiac. This lecture will review only skeletal muscle and will examine the mechanism of muscle contraction as it applies to movement. The body contains over 600 different skeletal muscles. These muscles perform three principal functions: (1) movement, (2) heat production, and (3) body support and posture.

Skeletal Muscle Anatomy

Myofiber or muscle fiber = muscle cell Endomysium = conn. tissue sheet around muscle cell Fascicle = bundle of muscles cells Perimysium = conn. tissue sheet around fascicle

Skeletal Muscle Anatomy Epimysium (fascia) = conn. tissue sheet around entire muscle Fascia can surround muscles and even muscle groups

Skeletal Muscle Cell or Fiber

Structure of Skeletal Muscle Cell Myofiber = muscle cell Sarcolemma  (muscle cell membrane) transmits the action potential (AP). Sarcoplasmic reticulum (SR)  the system of tubes controlling the release of Ca++, which is essential for contraction.  lacework of smooth E.R., stores calcium ions

Myofiber: Sarcoplasm  cytoplasm (very little) Myofibrils contain the contractile proteins of the muscle (thin and thick myofilaments) & are surrounded by a mesh-like network of tubes containing calcium ions (Ca++)

Myofiber: - myofibrils  cylindrical organelles, contains bundles of myosin (thick) & actin (thin) myofilaments Thick myofilament Thin myofilament MYOFIBRIL

Myofiber: transverse (T) tubules  narrow tube-like projections beginning at the cell membrane and extending into the cell  carries muscle AP deep into the interior where the contractile proteins are located  the S.R. becomes more permeable to Ca++ and diffuses into the sarcoplasm

Myofiber:  terminal cisternae or cisternae  a membranous enlargement of the SR, close to the T tubule where the action potential travels.  expanded endings of S.R., concentrated area of calcium ions (Ca 2+ )  mitochondria  produces ATP for muscle contraction

Myofiber:  - myoglobin  red pigment protein molecule in cytoplasm, similar to hemoglobin, stores O 2  stores O 2 temporarily  provides O 2 to muscle in times of inadequate blood flow  contributing factor to darker appearance of chicken leg or duck breast  associated with slow-twitch muscle fibers, resists fatigue and contract slowly

Skeletal Muscle Anatomy Whole muscles  bundles of fasciculi. Each fascicle  groups of muscle cells or fibers. Each muscle cell  many bundles of myofibrils. Each myofibril  thin and thick myofilaments. Thin myofilaments  mostly the protein actin along with troponin and tropomyosin. Thick myofilaments  protein myosin. It is this interaction of thin and thick myofilaments that result in muscle contraction!!!!

Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics of muscle/myofibers 1.Contractility 2.Excitability 3.Extensibility 4.Elasticity

Muscle Physiology Includes muscle contraction & muscle relaxation Requires: Nerve impulse = stimulus Neurotransmitter = acetylcholine (ACh), released by neuron and binds to muscle cell membrane (sarcolemma)  ATP = energy  needed for contraction & relaxation Calcium ions Acetylcholinesterase (AChE) = enzyme breaks down ACh

Stimulating a Myofiber Neurons stimulate myofibers (ie nerve impulse (AP) stimulate muscle cells)  Neuromuscular Junction = site where a neuron meets a myofiber, also called synapses

Neuromuscular junction (NMJ) NMJ= space between the muscle cell & neuron  Synaptic cleft or synapse- general term refers to a cell-to-cell junction (ie neuron to neuron or neuron to effector cell)

Neuromuscular junction Motor end plate = highly folded area of muscle cell membrane (sarcolemma). Increases stimulation of muscle cell

Excitation-Contraction Coupling

Muscle Mechanics Consists of 4 steps Excitation = nerve impulse, neurotransmitter & excitation of myofiber Coupling = ATP & calcium ions Contraction = muscle cells shorten Relaxation = muscle cell resume normal resting length (ATP)

Neuromuscular junction Neuron releases a chemical called Acetylcholine (ACh). The ACh diffuses across the synaptic cleft and then binds to sarcolemma (muscle cell membrane), stimulating it. ACh binds to receptor sites on sodium channels in the sarcolemma The combination or binding event leads to an increase in the permeability of the sarcolemma to sodium (Na+) Movement of Na+ into the muscle cell initiates an AP

Excitation Stage Where? At the neuromuscular junction, where a neuron meets (innervates) a myofiber (muscle cell) Neuromuscular Junction Structure Neuron = nerve cell conducts impulse Synaptic knob = neuron axon swollen ending Synaptic gap or cleft = small space between myofiber & neuron

Excitation Stage Neuromuscular Junction Structure Synaptic vesicles = small membranous sacs which contain neurotransmitter (acetylcholine) ACh receptors = location on sarcolemma where ACh binds when stimulating myofiber

Muscle Excitation Step 1: Nerve impulse travels down axon Step 2: As impulse reaches the synaptic knob, neurotransmitter, ACh is released Step 3: Neurotransmitter diffuses across synaptic cleft (gap) & binds to sarcolemma, stimulating it. Step 4: A muscle impulse is produced in sarcolemma

Muscle Mechanics Consists of muscle contraction and relaxation Requires: ATP and calcium (why?)

Myofibrils shorten  muscle cell contraction Thick & thin myofilaments slide across each other, causing myofibrils to shorten  muscle contraction (shorten)

Muscle Contraction Is due to the arrangement of the myofilaments (actin & myosin) in the myofibrils Let’s take a closer look at the myofilaments

Arrangement of myofilaments Sarcomere – smallest structural and functional unit of muscle. Many sarcomeres joined end to end form myofibrils

Mechanism of muscle contraction

What Happened? Did the myofibril become shorter? Did the thin & thick myofilaments become shorter? Any chemical bonding between thick & thin myofilaments?

Let’s Take A closer Look! Myofilaments: thick myofilaments = protein called myosin, resembles golf club (stick & head)

Myosin

Golf Club Head => myosin globular heads Contain ATP binding site, which binds ATP and breaks it down (hydrolysis) to ADP+ Pi. ATP energizes the globular head Golf Club Stick =>tail

Tail Globular Head ATP binding site MYOSIN ATP energizes myosin to change shape ATP

Let’s Take A Closer Look! Myofilaments: thin myofilaments = protein called actin, resembles golf balls

Myofilaments

Actin Golf Balls => strung together like beads Each contains myosin binding site, which binds myosin globular heads and forms a CROSS BRIDGE

ACTIN Myosin binding site on actin is not always exposed; otherwise myosin heads would bind constantly and our muscles would stay in a state of contraction (cramp!!!). Myosin binding site

ACTIN Muscle contraction is controlled in many ways. One is by exposing this myosin binding site on actin only when calcium ions leave S.R. and bind to the thin myofilament. Myosin binding site Ca 2+

ATP Ca2+

Copyright  The McGraw-Hill Companies, Inc. Permission required for reproduction or display. video

Sarcomere Shortening

Relaxation Action potentials cease Calcium no longer released from SR Calcium pumped back in to SR Up concentration gradient and requires ATP In absence of calcium, tropomyosin covers myosin binding sites again. Myosin becomes unable to bind actin Power stroke will not occur-> muscle relaxes

Changing the Force of Contraction Example: You are lifting weights at the gym. You pick up dumbells that differ in weight. In the left hand, you have the lighter dumbell and in the right hand the heavier dumbell. What is happening in each biceps muscle that allows you to pick up objects of different weight and control them in the same manner (ie when doing biceps curls)?

Altering the force of contraction….. Answer: You are able to alter the force of contraction to adjust to the weight being lifted through 2 mechanisms. 1. recruit motor units 2. summation of twitch contractions

video

Motor Unit -motor unit is a motor neuron and all of the muscle cells/fibers it causes to contract. -usually one motor neuron will contact (or innervate) several muscle cells, but each muscle cell is innervated by only one motor neuron. -large motor unit has a motor nerve in contact with a large number of muscle cells (up to roughly 200). -small motor unit is one in which the motor neuron contacts only a few muscle cells.

Recruitment Motor unit recruitment – the progressive activation of motor units resulting in a more forceful contraction

Summation of Twitch Contractions